1. Field of the Invention
The present invention relates to methods, apparatus and compositions for making metal objects. More particularly, the present invention relates to methods, apparatus and compositions for making three-dimensional metal parts by irradiating a mixture of metal powder and a vehicle which provides temperature equalization and unification and a protective atmosphere for the powder.
2. Description of Related Art
Because conventional casting and extrusion techniques often cannot efficiently and cost-effectively be employed to produce prototypes or small runs of parts, those skilled in the parts production art have sought to develop techniques better suited for such purposes. In general, the techniques developed by those skilled in the art to produce small numbers of parts can be classified in one of two separate categories, each of which is discussed below.
The first category comprises "subtractive methods". In a subtractive method, material is cut away from a starting piece of material to create a smaller, desired piece. Examples of subtractive machine tool methods include milling, drilling, grinding, lathe cutting, casting, machining, finishing and flame cutting.
The second category of techniques that may be employed to produce small runs of parts comprises "additive methods". Typically, additive methods involve adding material to a starting substrate to create a larger, desired piece. Examples of additive methods include plating, cladding, casting and certain welding processes.
Both of the aforementioned categories of techniques for producing small runs of parts have deficiencies and shortcomings. Deficiencies and shortcomings of subtractive methods are well known and have heretofore been expressed at some length, such as in the description of the relevant art section of U.S. Pat. No. 4,938,816 to Beaman et al. These deficiencies and shortcomings include the facts that they produce a large amount of waste material requiring disposal, they involve a large initial expense for setting up the proper machining and tools, and they involve tool wear that requires tool replacement and which reduces machining accuracy as the tool wears. Further, subtractive methods often simply cannot be employed to make a desired part. Symmetrical parts and parts where only the exterior of the part is to be machined can usually be easily produced by a subtractive method; however, where a desired part is asymmetrically shaped or requires internal machining, a part must often be broken into segments, which requires labor intensive assembly and bonding stages, in order for subtractive methods to be readily employed.
With respect to additive methods, a primary shortcoming and deficiency heretofore has been a general inability to produce a small, solid object. There has been, however, some recent activity directed towards overcoming this primary shortcoming and deficiency. In general, those skilled in the art have made efforts to use laser beams to coat or deposit material on a starting article. Such efforts are articulated in, e.g., U.S. Pat. Nos. 4,117,302, 4,474,861, 4,300,474 and 4,323,756. These relatively recent uses of lasers have been basically limited to adding a coating to a previous machined article. Typically, in such laser coating methods the starting article is rotated and the laser directed at a fixed location with the coating material sprayed onto the article so that the laser will melt the coating onto the article.
Still further with respect to additive methods of producing prototypes and small runs of parts, very recently certain persons skilled in the art have devised a method for building up parts from a powder by selectively sintering with a laser. The objective of this method has been to produce three-dimensional parts of a general shape directly from a CAD data base without part-specific tooling or human intervention. This method is described at length in U.S. Pat. Nos. 4,863,538, 4,938,816 and 4,944,817. Although the basic theory of this method appears to be sound, in practice this method has a number of serious shortcomings. For example, as powders are melted in air during practice of this method, there is little control over those powders and they tend to fly away uncontrolled and severly oxidized. Second, control of melting is relatively unrefined so that rough outlined shapes, rather than sharply defined shapes, are produced. Additionally, this method lacks refinements that make producing of metal parts, perhaps the most useful types of parts, practical. In summary, although this latter method of creating prototypes and small runs of parts has much promise, it has many shortcomings and deficiencies that render it yet relatively useless for certain applications, e.g., applications involving production of solid metal parts.